Active source seismic reflection data are commonly used in hydrocarbon exploration to remotely infer subsurface geologic structure and rock properties. A major advantage of these data are that the source and receiver locations can be strictly controlled, thereby allowing very detailed imaging of the subsurface. However, reflected signals from strata are recorded as time series, and must be migrated to depth in order to infer geologic structure. This process requires a reliable seismic velocity reference model, whose accuracy directly impacts the accuracy of the inferred structure.
A common approach to building migration models is to perform Normal Move-out Analysis or travel-time analysis on the seismic reflection data [e.g., Sheriff and Geldart, Exploration Seismology, Cambridge University Press, 134-135 (1982)]. In complex regions such as fold-thrust belts, regions with basalt sills and dykes, or intruding salt bodies, the quality of these models is often poor, leading to poor migration of seismic reflections and therefore inaccurately inferred structure.
The present invention is a new method for using seismic waves to obtain a subsurface velocity model for use in hydrocarbon exploration. Seismic energy may be generated either actively (e.g. by explosions or vibrations) or passively (e.g. by earthquakes or landslides). Seismic energy in the Earth travels in the form of either compressional waves (P waves) or shear waves (S waves). P waves can be identified by the presence of particle motion in the direction of wave propagation; whereas S wave particle motion is perpendicular to the direction of wave propagation (FIG. 1). P and S waves travel at different speeds through the Earth, and these speeds are important indicators of subsurface properties such as lithology, porosity, or fluid content.
FIG. 2 is a schematic diagram of the process of seismic exploration. Seismic waves can be observed at any geographic location using a purpose-built recording instrument (for example, a geophone or seismometer). These instruments are designed to record particle displacements or acceleration, and require three independent (typically orthogonal) components to fully describe particle motion at the Earth's surface.
Seismic energy observed at a deployed instrument (commonly referred to as a station) is recorded in the “station reference frame.” For consistency, stations are usually carefully positioned and leveled in the field such that the station reference frame corresponds to a standard geographic reference frame, with components pointing in the North (N), East (E), and vertical (Z) directions.